Field of the Invention
This invention relates to a hybrid gas generating system for the inflation of an inflatable restraint for passengers in a vehicle such as an automobile, a boat, or an airplane. More particularly, it relates to a novel, chlorine-free gas generant which utilizes an extrudable thermosetting binder whose combustion products are essentially free of nitrogen oxides.
As is well known in the inflatable restraint art, compressed gas may be utilized to inflate an air bag or similar safety cushion in a moving vehicle in the event of a sudden deceleration of the vehicle, such as that caused by a collision, for the protection of a passenger in the vehicle. Such compressed gas may be the only inflating material or its action may be augmented by the heat and gas generated by the combustion of a fuel in a heater cartridge which is adapted to communicate with a chamber containing said compressed gas. Similarly, various pyrotechnic compositions have been proposed for generating a gas upon combustion in order to serve as the sole inflating agent of an air bag or to augment a compressed gas. Exemplary of the many patents issued in this field are U.S. Pat. Nos. 3,692,495 (Schneiter et al); 3,723,205 (Scheffee); 3,756,621 (Lewis et al); 3,785,149 (Timmerman); 3,897,285 (Hamilton et al); 3,901,747 (Garner); 3,912,562 (Garner); 3,950,009 (Hamilton); 3,964,255 (Catanzarite); 4,128,996 (Garner et al); 4,981,534 (Scheffee); and 5,290,060 (Smith), which is incorporated herein by reference.
The use of compressed gas as the sole inflating agent is subject to a variety of disadvantages such as bulkiness of the container which makes it difficult to store in places such as the steering wheel or dashboard of a car. Also, the pressure in the container may rise to undesirable levels along with the ambient temperature. Moreover, the response time of a system using compressed solely is unacceptably slow. On the other hand, several criteria must be met by a pyrotechnic gas generant to be satisfactory for inflatable restraint systems. It must produce non-toxic, non-flammable and smokeless gas over a wide range of temperatures and other environmental conditions. The temperature of the generated gases must be sufficiently low that they may be cooled further by the conventional coolant techniques known in the art so as not to destroy the air bag or injure the passenger. The pyrotechnic must be safe to handle and must be capable of generating a very large amount of gas within a very short time frame, i.e., about 35 milliseconds.
Sodium azide-based compositions are the current leaders in all-pyrotechnic inflation systems both driver side and passenger side installations because of their excellent gas generating properties and the non-toxic nature of the nitrogen gas produced. Passenger side installations require much larger volumes of gas, however, and hybrid systems are being turned to in order to satisfy that requirement. The two Scheffee patents mentioned above teach the use of a PVC plastisol [poly (vinyl chloride) plus a plasticizer] as a fuel and binder for the pyrotechnic material in a hybrid gas generator. The presence of the poly (vinyl chloride) requires a PVC stabilizer. It also requires a chlorine scavenger to prevent the passage of toxic chlorine or hydrogen chloride gas into the air bag and thence into the passenger compartment. Thus, in addition to the binder, plasticizer, stabilizer, and oxidizer, the pyrotechnic material must contain an alkali- or alkaline earth metal salt and may contain carbon, iron oxide, and a transition metal oxide. This makes a complex system.
To replace sodium azide, Garner et al teaches polyacetal and poly (vinyl acetate) resins as fuel for the gas generating combustion in an air bag inflator. The resin and oxidizer are milled in a solvent, then dried and pressed into pellets. Lewis et al teaches the use of argon as the compressed gas and a poly vinyl composite or other material as the gas generating combustible material in a hybrid system. Schneiter et al teaches that a solid fuel for air bag inflators may be made by curing a mixture of a liquid carboxyl-terminated polyester, a diglycidyl ether of bisphenol A, potassium perchlorate, aluminum oxide, and a catalyst for 72 hours at 135.degree. F.
In order for formulations containing thermosetting binders to be extrudable, several conditions must be satisfied. Among them are:
The viscosity of the formulation must be high enough when it exits the extruder that the extrudate will hold its shape until curing is complete. This means that the mix viscosity must be high enough that curing reactions in the extruder are unnecessary; or PA1 If the uncured composition does not have that requisite viscosity, the cure chemistry of the formulation must allow at least partial curing within the extruder to control the exudate viscosity so that it may be formed into a grain with controlled dimensions and which will retain them while full cure is progressing; and PA1 Rapid curing reactions within the extruder are undesirable; rapid curing and the consequent plugging and overheating must be avoided.
In an article entitled "Studies on Composite Extrudable Propellant with Varied Burning Rate Pressure Index `n`", Def. Sci. J., Vol. 39, No.1, January 1989, pp 1-12, T. L. Varghese et al teach that the evaporation of process solvents creates porosity, internal cracks, and dimensional variation during solvent extrusion of propellants. Citing the better physical and mechanical properties of extruded cross-linked composite propellants, along with better aging characteristics, dimensional stability, and better ballistics control, Varghese et al described a propellant comprising a carboxyl-terminated polybutadiene, ammonium perchlorate, and a diepoxy-triaziridine combination as the curing agent. The proper consistency for successful extrusion was achieved only after seasoning the thermosetting propellant mix at 60.degree. C. (140.degree. F.) for six hours.